Using the standard mounts there isn't mush room for spacers behind the grille. The Alpines shouldn't need a spacer but I'd get the mounting depth needed and double check first. For the cash though, I'd look at getting a set of JL TR's though......
As for rears, I'd put most of my budget up front. If you want the music to sound as it intended, you need the sound up front. So probably faded forward a bit and therefore you don't need the best speakers behind you. Put the effort into the front speakers and you'll heard the benefits
Factory sub mount will need a lot of effort to seal it well enough to do a proper job with a decent sub. It needs a bit of research as there's only so much space behind the seat and subs need a certain volume to work best in.